To effectively convey electrical power from one location to another, an electrical cable must minimize or prevent any degradation in its conductivity caused by moisture. High moisture levels inside the electrical cable can have a detrimental effect on the conductive characteristics of the cable. Depending on the construction of the particular electrical cable, the introduction of moisture can result in a short circuit, an increase in the electrical cable's capacitance, or in the complete failure of the electrical cable.
Moisture can penetrate to the interior of the electrical cable by several different ways. Water may enter through a failure in an electrical cable's jacket. Water may also enter through a failure where two electrical cables are joined. Mechanical impacts, electrical arcs, or lightning may breach the jacket that protects the electrical cable or the joint where the electrical cable joins another. Water may then flow towards the core of the electrical cable and longitudinally along the length of the electrical cable. Changes in ambient conditions may lead to differences in water vapor pressure between the interior and the exterior of the electrical cable. The difference in vapor pressure then causes moisture to diffuse into the interior of the electrical cable. Eventually, there may be an undesirable level of moisture inside the cable. Since the electrical cable's ability to resist penetration by moisture may be a crucial characteristic in certain applications, the electrical cable must be tested and meet specific standards to minimize the possibility of conductive degradation caused by the presence of water within the electrical cable.
Different standards with respect to water blocking apply to electrical cables depending on the particular application and use. One such standard is MIL-DTL-24643, which is set by the Naval Sea Systems Command, and it specifies the water blocking requirements for an electrical cable to be used on a Navy ship. To meet the requirements of MIL-DTL-24643, an open end of the electrical cable is subjected to a predetermined water pressure for a predetermined amount of time. Electrical cables that allow a specified minimum amount of water migration to a specified length when subjected to the test are deemed “water blocked.”
Various methods have been used to block water. A pulp or paper insulated electrical cable tends to swell and prevent further penetration of moisture. But, once water enters the interior of the electrical cable with a pulp or paper insulation, it can cause short circuits between conductors. In a cable insulated by plastic or a polymeric material, water can travel by capillary action along the cable interstices, causing problems in conductivity. In most environments, it is desirable, if not essential, that the cable is more watertight than can be achieved with polymeric material alone. Some electrical cables may include a metal/plastic laminate foil beneath the outer protective jacket of the electrical cable. The metal/plastic laminate foil may become bonded to the polymeric material, normally when the polymer is extruded. However, it is difficult to design a jacket in which the laminate foil remains intact when the electrical cable is subjected to impact, as the laminate tends to be driven into gaps between conductors lying underneath the laminate and to split along resulting crease lines.
Another method of protecting a cable against water penetration by capillary action along cable interstices is to use a filler material. Filler materials are commonly synthetic polymers, petroleum based greases, oils, or silicone flooding compounds. Filler materials may be coated on components of the electrical cable to prevent longitudinal movement of moisture. Also, interstices within the cables may be filled with the filler material to minimize water entry and migration. However, filler materials such as silicone flooding compound do not consistently block water. Also, applying filler material in order to block water necessitates additional handling and processing measures in the manufacturing of the cable. The additional measures increase manufacturing time. Further, the addition of filler material significantly increases the weight of the electrical cable. Finally, electrical cables with filler material cannot be terminated without peeling away or removing waterblocking material on the insulated conductors.